This invention relates generally to the field of gravure printing and specifically to an improved doctor blade.
Photogravure or gravure printing is a technique whereby intaglio engravings of an image which is to be printed on a substrate (usually paper) are formed by known techniques on the surface of a gravure cylinder. Intaglio engravings are those where the elements to be printed are below the surface of the gravure cylinder, having been cut or etched into the metallic cylinder to form ink-retaining grooves or cups.
It has long been recognized that a very important factor in producing high quality gravure printing is the proper design and use of a doctor blade.
A doctor blade is a long, thin strip of metal which is mounted on a gravure printing machine parallel to the axis of the printing cylinder, with the thin edge of the strip very close to the surface of the gravure cylinder. When the cylinder is in operation, the doctor blade controls the amount of ink transferred from the printing surface of the cylinder to the paper or other impression-receiving material by wiping of the surplus ink.
Examples of doctor blades illustrated in the prior art are as follows:
______________________________________ U.S. Pat. No. 1,092,798 U.S. Pat. No. 1,760,049 U.S. Pat. No. 2,230,583 U.S. Pat. No. 2,313,830 U.S. Pat. No. 2,361,554 U.S. Pat. No. 2,655,561 U.S. Pat. No. 3,040,442 U.S. Pat. No. 3,122,767 U.S. Pat. No. 3,521,561 U.S. Pat. No. 3,649,991 U.S. Pat. No. 3,720,163 U.S. Pat. No. 3,866,266 U.S. Pat. No. 3,884,145 U.S. Pat. No. 4,151,797 U.S. Pat. No. 4,184,429 Canadian Patent No. 986,783 Canadian Patent No. 991,477 Dutch Patent No. 7,312,922 W. German Patent No. 2,305,120 W. German Gebrauchmuster 1,976,227 Japanese Patent No. 5,121,060 Japanese Patent No. 6,101,853 ______________________________________ "Doctor Blade Problems," by Heinz Sander, 1966 Rotogravure Yearbook, Gesellschaft zur Forderung des Tiefdrucks e.V., Wiesbaden, Germany.
Throughout the long history of gravure printing, it has been recognized that, while there are many factors affecting the quality of gravure printing, one of the most important, if not the most important, is the proper design and usage of the doctor blade. Specifically, even if a gravure cylinder has been properly etched and cut, improper doctor blade design and usage will prevent the obtaining of a proper image.
It is highly desirable that there be uniform pressure over the width of the doctor blade against the gravure cylinder in the printing areas so that there is a clean, even wipe with resulting even print quality. It is also desirable that there be uniform pressure across the width of the doctor blade because with uneven pressure across the width of the doctor blade there is uneven doctor blade and cylinder wear, with the greatest wear occurring where the doctor blade is stiffest. The uneven doctor blade pressure discussed herein is a problem in the prior art and one which the present invention is, inter alia, aimed at solving.
A further problem with prior art doctor blades results from doctor blade camber--i.e., the convexity of the working edge of the doctor blade. Camber results in the doctor blade wearing unevenly, with concomitant uneven wear at the center of the gravure cylinder which the doctor blade is wiping. Camber has heretofore been a problem with prior art doctor blades. However, it should be noted that the camber at the end of the doctor blade is beneficial in that it reduces the force applied to the doctor blade, and hence wear, where it is only partially supported by the cylinder.
A further problem with prior art doctor blades has been bleeding. Bleeding may be defined as the depositing of an ink film or ink particles on non-depressed areas of a gravure cylinder adjacent the cut-out or etched areas of the gravure cylinder. The result is a spillover on the printed substrate such that adjacent the trailing edge of a solid printed section where there is intended to be a non-printed section ink particles appear. This, of course, is highly undesirable and lowers print quality.
A further problem with prior art doctor blades is end wear. More specifically, this problem arises from two principal causes. One of the causes is that the edges of the doctor blade overlay the cylinder and do not receive adequate lubrication and support. Another cause is the lateral oscillation that is inherent in the operation of a doctor blade. Obviously end wear is undesirable in that it adversely affects doctor blade life.
A further problem associated with many prior art doctor blades was the presence of dry or liquid ink particles between the backup blade and the doctor blade. This resulted in the backup blade not uniformly supporting the doctor blade during doctoring operations, causing the spring constant of the doctor blade assembly to vary, which, of course, is undesirable.
Uniform pressure against the entire printing cylinder by the doctor blade is not desirable in that uniform pressure is desirable only against those portions of the printing cylinder which do the printing. At the end portions of the doctor blade, where printing does not occur, it is desirable to relieve pressure. Sometimes in the prior art the backup blade was cut back to relieve pressure at the end of the printing cylinder.
While I have discussed herein technical factors affecting print quality and doctor blade and cylinder life, the economics of running a gravure printing press operation must, of course, be considered. Gravure press equipment is universally recognized as being expensive. In addition, the labor costs associated with running a gravure press are also significant. This is because gravure pressmen are highly skilled and as a consequence thereof are highly paid. Thus, it can readily be seen that anytime a gravure press is not operating during a period when it is supposed to be producing printed substrate (downtime) there are significant costs associated with this downtime. In addition, downtime may result in failure to meet printing deadlines.
A frequent cause of press downtime is the need to replace worn doctor blades. A doctor blade generally has a certain useful life that will vary with press conditions, which useful life is measured in revolutions of the gravure cylinder. Thus, a particular doctor blade for a particular set of press conditions may have a working life of 200,000 revolutions of the gravure cylinder, etc. When a doctor blade needs to be replaced, then the press must be stopped, resulting in the expensive press machinery being inoperative. Naturally, the prior art has made efforts to extend doctor blade life.
Another factor that must be weighed in the economic analysis of a pressroom operation is the cost of a doctor blade. Heretofore, doctor blades were made in their entirety of doctor blade steel. Doctor blade steel is relatively expensive and any reduction in the amount of doctor blade steel in a doctor blade will affect pressroom economics.